Efficient optical projection of patterns is used in diverse applications such as optical three dimensional (3D) mapping. Such a mapping generates a 3D profile of the surface of an object by processing its optical image. The desired properties of the projected patterns generally depend on the application, but typically include high contrast, high projection efficiency and high uniformity of intensity distribution between the projected beamlets that make up the pattern. The uniformity of the intensity distribution can be expressed in terms of the beamlet-peak-to-average power ratio, i.e., the ratio of the intensity of the strongest beamlet to the mean beamlet intensity, wherein a low ratio corresponds to high uniformity.
Poor uniformity of a projection method may render it unusable for certain applications, such as those applications in which the projected pattern can be seen by the user. Typically, in these cases eye safety regulations dictate the maximal permitted energy flux per unit cross-sectional area or, alternatively, the maximal permitted energy flux in a single collimated beamlet.
Patterns may be projected using diffractive optical elements (DOEs). However, the DOEs may suffer from the so-called zero order problem. A zero order beam is the portion of the incident beam that is not diffracted by the projection setup and thus continues through the system onto the projection volume.
U.S. Pat. No. 6,560,019, to Nakai, whose disclosure is incorporated herein by reference, describes a diffractive optical element having a first and a second grating. The diffraction efficiency of the element is stated to be 97% or better throughout the entire visible spectrum.
U.S. Patent Application 2007/0019909, to Yamauchi et al., whose disclosure is incorporated herein by reference, describes an image display device which included a diffractive optical element and a first “face.” The face is provided at a position on which zero order light emitted from the diffractive optical element is not incident, and an image is displayed by light via the first face.
Notwithstanding the above art, an improved method for projecting patterns would be advantageous.